The present invention relates to a process for the preparation of low-viscosity polyisocyanate reaction products which contain allophanate groups, can be readily emulsified in water and contain activated groups which react, by polymerization, with ethylenically unsaturated compounds under the action of actinic radiation. The present invention furthermore relates to the products which can be prepared by the process according to the invention and their use.
The curing of coating systems carrying activated double bonds by actinic radiation is known and is established in industry. Actinic radiation is understood as meaning electromagnetic, ionizing radiation, in particular electron beams, UV rays and visible light (Roche Lexikon Medizin, 4th edition; Urban & Fischer Verlag, Munich 1999). It is one of the fastest methods of curing in coating technology. Coating compositions based on this principle are therefore called radiation- or actinic-curing or -curable systems.
Due to the ecological and economic requirements of modern lacquer systems of using as little organic solvent as possible or even no organic solvent to adjust the viscosity, there is the desire on the one hand to use lacquer raw materials which are already low-viscosity and on the other hand moreover to be able to carry out necessary viscosity adjustments with water as the solvent.
Particularly low-viscosity, radiation-curing binders are, as is known, urethane acrylates containing allophanate groups. These can be prepared by various processes, thus e.g. by direct reaction with an excess of diisocyanate and subsequent distillation of the excess diisocyanate (EP-A 0 867 457 or WO-A 00/39183), by opening of an oxadiazinetrione (WO-A 2004/033522), by opening of a uretdione (WO-A 2005/092942) or also by direct allophanation of an equimolar ratio of isocyanate groups to urethane groups (EP-A 1 645 582). Only the application mentioned last also discloses the possibility of arriving at water-emulsifiable binders via a hydrophilization. Nevertheless, the products obtained in this way are not particularly stable in the emulsion and a phase separation already occurs after a short time.
Urethane acrylates which contain allophanate groups and are water-emulsifiable due to incorporated hydrophilizing agents have also already been disclosed. EP-A 0 694 531 thus describes a method for the preparation of such binders. Nevertheless, a minimizing of the viscosity is relinquished here by the choice of an ionic hydrophilization. Furthermore, it is a complicated multi-stage process, which must be carried out at very high temperatures of more than 100° C., which is detrimental to the stability of systems with activated double bonds. Finally, the process is not aimed at low-viscosity 100% systems, and finished emulsions are already obtained.
WO-A 2007/063025 likewise describes a water-emulsifiable urethane acrylate containing allophanate groups. Nevertheless, these allophanate structures are not produced in the process disclosed, but are incorporated via a modified isocyanate. Such modified isocyanates can be prepared only with difficulty and expense, since the process described in WO-A 00/39183 comprises a distillation process which can be transferred to industry only with difficult. In the end, isolation of an intermediate product is thus necessary.
It was therefore the object of the present invention to provide a simple process for the preparation of a radiation-curable polyurethane (meth)acrylate which, on the basis of allophanate structures, is of particularly low viscosity as an undiluted system with a solids content of 100 wt. % and can be emulsified readily by stirring into water. Furthermore, the water-diluted radiation-curable polyurethane dispersion should be storage-stable.